An essential prerequisite for the production of cut surfaces having a high surface quality and dimensional uniformity is a cutting tip free of slag spatter and contamination. It is known that in autogenous cutting, the beginning of the cut must be heated to ignition temperature so that the exothermal reaction required for the cutting process is attained. For this purpose, the cutting oxygen bore is usually surrounded by a crown of heating flames.
The cut is usually started on the stationary workpiece from one workpiece edge whereby the cutting torch is moved to the edge and the heating flames heat the site of the edge to ignition temperature. Depending on the thickness of the workpiece, this process takes several minutes. When the ignition temperature has been reached, the cutting oxygen is turned on and the movement of the torch is started.
In an acetylene-oxygen flame, the hottest place is found just in front of the peak of the brilliant flame cone. The distance of the cutting tip from the workpiece surface should, therefore, only be such that the peaks of the flame cones almost touch so that the heat capacity can be fully utilized. At the start of the cutting process, the torch is raised to prevent adhesion of spatters.
In longitudinal cutting of castings continuously emerging from continuous installations, the problem occurs that at the beginning of the casting the starting piece is in contact and the workpiece is moving so that the cut must be started with the socalled hole punching process. The cutting torch with its heating flames is then closely arranged above the casting, as described above, and the casting surface is heated to ignition temperature. When this temperature has been reached, the cutting oxygen valve is slowly opened up to the required maximum pressure and the torch is simultaneously raised as a function of the thickness of the sheet. The cutting stream penetrates the sheet and the slag spatters upward until the cutting stream emerges from the bottom of the casting.
Since the described hole punching process requires some time, the torch cannot be arranged stationary but during hole punching must be moved along with the casting. This represents apparative expense and an extended roller bed.